Recently, a vehicle having an idle stop function to automatically stop an engine when the vehicle is stationary because of waiting for a traffic light or the like has been put into practical use for purpose of improvements in fuel economy and environmental performance. Moreover, in the vehicle having such an idle stop function, an automatic transmission in which a continuously-variable transmission mechanism (continuously-variable shift mechanism, hereinafter referred to as “variator”) and a stepped transmission mechanism (stepped shift mechanism, hereinafter referred to as “subtransmission mechanism”) are disposed in series with each other is mounted. (for example, see Patent Literature 1)
In the vehicle disclosed in Patent Literature 1, an interlock state in which rotation of an output shaft of the subtransmission mechanism is fixed is realized by fully engaging a first clutch and a second clutch under an idle-stop control (i.e. during engine stop). The first clutch is configured to be engaged when the vehicle starts moving, and the second clutch is provided separately from the first clutch. Moreover, when the idle-stop control is terminated, the subtransmission mechanism is made to be in a slipping interlock state by bringing the second clutch into a slipping state instead of the fully-engaged state while keeping the first clutch in the fully-engaged state. Accordingly, when the idle-stop control is terminated, a push-up feeling of a driver can be reduced and hence a strangeness feeling of a driver can be reduced, according to teachings of Patent Literature 1.
In general, the idle-stop control is executed to automatically stop the engine in a situation where a vehicle speed has become equal to 0 by brake manipulation and in a situation where the vehicle does not start moving by itself even if the engine is stopped. In the technique of Patent Literature 1, the subtransmission mechanism is controlled to become in the interlock state when the vehicle speed becomes equal to 0 by the brake manipulation. At this time, it is desirable that a hydraulic pressure which reliably prevents the vehicle from moving is supplied to the clutch which is engaged for the interlock, according to teachings of Patent Literature 1. Accordingly, for example, even if the vehicle is stationary on a sloping road, the vehicle is kept not to start moving by its own weight during the interlock. The interlock is released when a brake switch becomes in an OFF state, and hence the vehicle can start moving when the brake switch becomes in the OFF state.
A case can sometimes occur that the subtransmission mechanism becomes in the interlock state because of an ON state of the brake switch and a zero state of the vehicle speed and then the vehicle starts moving again without automatically stopping the engine. In such a case, the interlock needs to be promptly released to start the vehicle, when the brake switch becomes in the OFF state before the stop of the engine.
However, the brake switch is normally switched between the ON state and the OFF state according to a level of brake fluid pressure. Hence, the brake fluid pressure is sometimes insufficient even when the brake switch is in ON state. For example, if a depressed brake pedal is slowly released, the brake fluid pressure is gradually reduced and thereby a braking force sometimes becomes insufficient although the brake switch is in ON state. That is, a gap is sometimes caused between an actual braking force and an ON/OFF information of the brake switch.
Thus, in the case that the actual braking force is insufficient although the brake switch is in ON state, the vehicle is supported by the interlocked subtransmission mechanism. For example, in the case that the vehicle is stationary on a sloping road, torque applied to road-wheels is inputted through a driveshaft into the subtransmission mechanism. In this state, if the brake fluid pressure further decreases such that the brake switch is changed from the ON state to the OFF state, the interlock is released. Hence, the torque inputted into the subtransmission mechanism is released in one shot, so that torque is rapidly transferred in the automatic transmission with a torsional motion of the driveshaft. These rapid torque transfer and torsional motion of the driveshaft influence a behavior of the vehicle as a shock. As a result, a feeling of knocking (pushing) in a frontward direction is given to a driver.
In order to avoid this shock, it is conceivable that the interlock is gradually released from a time point when the brake switch is changed from the ON state to the OFF state. However, if the release of the interlock takes plenty of time, a startability of the vehicle is worsened at the time of release of the brake pedal. Therefore, it is desired that the startability is ensured at the time of release of the brake pedal while suppressing the shock which is caused due to the release of the interlock (i.e. at the time of movement start of the vehicle).
The existence of such a problem is not limited to the subtransmission mechanism provided to the variator. A similar problem exists also against a general stepped automatic transmission mechanism as long as this transmission mechanism is configured to become in the interlock state when the vehicle is stationary.